Aerodromes - Visual Aids, Markings and Signs — Page 411, Lesson 562

I want to walk you through the next set of signs and markings on the aerodrome, starting with how taxiways are named. First, taxiway designators. Every taxiway gets a designator made up of a letter, or a combination of letters, or a letter followed by a number. So you might see a taxiway called "A," or "B4," or "C12." Now, there's a rule about which letters to avoid: the letters I, O, and X should be avoided wherever possible. Why? Because I looks like the numeral 1, O looks like the numeral 0, and X is used for closed markings. You don't want a pilot to misread a taxiway letter as a number or as a closed runway. Also, numbers alone on the manoeuvring area are reserved for runways. So if you see a number by itself on the movement area, that's a runway designator, not a taxiway. Next, aircraft stand identification signs. An aircraft stand has an identification marking, which we'll cover later in paragraph 20.55, and where feasible, that marking should be supplemented with a sign. The sign must be located so it's clearly visible from the cockpit of an aircraft before entering the stand. The sign itself has an inscription in black on a yellow background. Alternatively, in Europe, it may be white on a blue background. So two colour schemes are acceptable. Now, road-holding position signs. These are provided at all road entrances to runways. The sign is located 1.5 metres from the edge of the road, on the left or right as appropriate to local traffic regulations, at the holding position. The sign consists of an inscription in white on a red background. If it's intended for night use, it must be reflective or illuminated. The inscription must be in the national language, must conform to local traffic regulations, and must include three things: a requirement to stop, a requirement to obtain an ATC clearance, and the location designator. So the sign tells the driver to stop, to get an ATC clearance, and identifies where they are. Finally, illumination of signs. Signs must be illuminated when intended for use in two conditions: first, in RVR conditions less than 800 metres — that's runway visual range below 800 metres; and second, at night. The excerpt cuts off mid-sentence there, but the principle is clear: if you're going to use a sign in low visibility or darkness, it has to be lit. Let me tie these together. Taxiway designators give you a naming system that avoids confusion with numbers and closed markings. Stand identification signs help you find your parking spot from the cockpit. Road-holding position signs control vehicles entering runways, with a strict stop-and-clearance requirement. And illumination ensures all of these remain usable when visibility drops or at night.